1. Field of the Invention
The present invention relates to a method and an apparatus for measuring wavelength dispersion value and/or nonlinear coefficient of optical fibers.
2. Prior Art
Recently, accompanied with explosive increase in data communication such as internet, rapid increase of data transmission capacity has been required. At present, wavelength division multiplexing (WDM) transmission system which transmits simultaneously a plurality of signal lights differed slightly in wavelength through an optical fiber has been used practically, and applied to a trunk transmission line and an undersea optical cable. According to this, enlargement of transmission capacity has been progressed rapidly, however, increase of wave number and increase of bit rate of signal light have been caused various problems. For example, different dispersion is occurred at both ends of using wavelength band width by inherent wavelength dispersion value of an optical fiber, therefore, problems of wave form deterioration after transmission and of nonlinear phenomenon are caused. The nonlinear phenomenon is caused by change of reflective index of an optical fiber constituting a transmission line, and four wave mixing (FWM) corresponds to this phenomenon. Especially, the problem is serious in case of a plurality of different wavelength signal lights are transmitted at long distance such as WDM transmission. These dispersion and nonlinear phenomenon may become main factor which deteriorates transmission quality. In order to solve these problems, development of an optical fiber suppressing nonlinear phenomenon is required, and accompanied with this, establishing a method for measuring nonlinear refractive index or nonlinear coefficient is required. Nonlinear refractive index n2 is a characteristic of an optical fiber constituting material, and nonlinear coefficient N2 is a characteristic of an optical fiber, by defining an effective core area of an optical fiber is Aeff, relation between n2 and N2 is expressed as equation (1).
N2=(n2/Aeff)xe2x80x83xe2x80x83(1)
According to equation (1), if effective core area Aeff is known, nonlinear refractive index n2 is calculated from nonlinear coefficient N2, therefore, method for measuring nonlinear refractive index n2 and method for measuring nonlinear coefficient N2 are substantially the same.
As a method for measuring nonlinear coefficient of an optical fiber, a method of emitting a pulse light to an optical fiber from a pulse light sauce, measuring phase shift from change of power spectrum wave form caused to signal light by self phase modulation, and obtaining nonlinear refractive index has been reported (R. H. Stolen and Chinlon Lin, Physical Review A, vol. 17, no. 4, p. 1448-1453 (1978)). This method is generally called self phase modulation (SPM) method.
Further, a method of emitting probe light and pump light modulated sinusoidally at predetermined frequency to an optical fiber, detecting probe light by delayed self heterodyne detecting, measuring phase shift of probe light caused by pump light, and obtaining nonlinear refractive index has also been reported (A. Wada et al, ECOC 92, p. 45 (1942)). This method is generally called mutual phase modulation (XPM) method.
Further more, a method of emitting two continuous wave (CW) light to an optical fiber, measuring optical intensity ratio between first siderove wave generated by SPM effect and measuring light, and obtaining nonlinear coefficient from relation between average input strength and optical intensity ratio has been reported (Boskovic et al, Optics Letters, Vol. 21, no. 24, p. (1996)). This method is called CW-SPM method, it became to be used mainly at present.
FIG. 1 is a block diagram showing measuring apparatus using CW-SPM method.
Signal lights from CW light source 1, 2 are arranged polarization state by polarization controller 3, 4, and coupled by optical fiber coupler 5. The coupled signal light is amplified by an optical amplifier 6 such as, for example, erbium (Er) added optical fiber amplifier. At this tome, since noise level is also amplified, noise level is lowed by bandpass filter 7. The signal light is amplified again by an amplifier 8, and emitted to a test optical fiber 10 through an optical fiber coupler 9. Wave form spectrum emitted from a test optical fiber 10 is measured by optical spectrum analyzer 13.
The optical fiber coupler 9 is to distribute emitted light, a power meter 12 is to measure emitted power, and a power meter 11 is to consider attenuation of emitting power by measuring rear Brillouin scattering.
FIG. 2 shows observed wave form measured by apparatus shown in FIG. 1, the horizontal axis showing wavelength and the vertical axis showing light intensity.
Measuring optical power intensity I0, I1 as shown in FIG. 8, and substituting obtained data for equation (2) and equation (3) with input power P measured by the power meter 12, n2/Aeff is obtained.
(I0/I1)={J02(xcfx86SPM/2)+J12(xcfx86SPM/2)}/{J12(xcfx86SPM/2)+J22(xcfx86SPM/2)}xe2x80x83xe2x80x83(2)
xcfx86SPM=(4xcfx80n2/xcexAeff)LeffPxe2x80x83xe2x80x83(3)
Where Jn is the first kind Bessel function, xcfx86SPM is the maximum phase shift and Leff is effective length of the test optical fiber 10.
In a conventional method for measuring nonlinear coefficient of optical fibers, there is a problem that high accuracy measurement is difficult.
In SPM method, nonlinear coefficient is determined by relation between maximum phase shift xcfx86SPM obtained by comparing output power spectrum wave form with theoretical wave form and optical power.
However, theoretical spectrum wave form shows time wave form of emitting light pulse being Gaussian as well as wave form having phase shift xcfx86SPM=0 to pulse peak, it is obtained by assuming condition that frequency chirp is not exist and can ignore effect of group velocity distribution of optical fiber. Therefore, in order to generate pulse light closing to an ideal, complicated adjustment work is needed.
Since, frequency chirp is included in actually generated optical pulse, generating complete ideal wave form being impossible, and difference from theoretical spectrum wave form being arisen, therefore, it is difficult to raise measurement accuracy.
XPM method is an indirect method for improving problem that SPM method is difficult to generate ideal pulse light. However, in XPM method, phase shift depends on relative polarization state of pump light and probe light, in a measurement of usual optical fiber which does not maintain polarization state, operation to take average of many polarization state is necessary to raise measurement accuracy. Further, because of indirect measuring method using two wavelength signal lights, measuring apparatus and analysis of measured data are complicated. Further more, since effect of group velocity distribution of optical fiber is ignored as well as SPM method, group velocity distribution of optical fiber may actually influence to measuring result.
Also in CW-SPM method used mostly at present, since effect of group velocity distribution of optical fiber is ignored, obtained nonlinear coefficient is differed greatly by emitting power at measurement, length of a test optical fiber, wavelength interval of two CW light sources, and wavelength dispersion value of light source wavelength.
It is pointed out that these three methods do not give coincident measurement results to the same optical fiber.
An object of the present invention is to provide a method and an apparatus for measuring wavelength dispersion value and/or nonlinear coefficient of optical fibers which realize high accurate nonlinear coefficient and wavelength dispersion value without influence of emitting power at measurement, length of test optical fiber, wavelength spacing of light source, and varying wavelength dispersion value of light source.
In accordance with the present invention, there is provided a method for measuring wavelength dispersion value and/or nonlinear coefficient of optical fibers comprising, a process for emitting a pump light emitted from a first continuous wave light source and a signal light emitted from a second continuous wave light source to a test optical fiber by varying peak wavelength spacing between said pump light and said signal light, and measuring four wave mixing optical intensity generated in said test optical fiber at every peak wavelength spacing, a process for measuring optical intensity of a plurality of line spectrum constituting said pump light and a plurality of line spectrum constituting said signal light respectively, and a process for calculating so that maximum value of four wave mixing optical intensity in each line spectrum wavelength spacing calculated from said measured optical intensity of line spectrum of said pump light and said signal light coincides with measured value of optical intensity of four wave mixing in each peak wavelength spacing, and obtaining nonlinear coefficient and/or wavelength dispersion value.
Further, there is provided a method for measuring wavelength dispersion value and/or nonlinear coefficient of optical fibers comprising, a process for emitting a pump light emitted from a first continuous wave light source and a signal light emitted from a second continuous wave light source to a test optical fiber by varying peak wavelength spacing between said pump light and said signal light, and measuring four wave mixing optical intensity generated in said test optical fiber at every peak wavelength spacing, a process for measuring optical intensity of a plurality of line spectrum constituting said pump light and a plurality of line spectrum constituting said signal light respectively, and a process for calculating so that maximum value of four wave mixing optical intensity P1 (L) in each line spectrum wavelength spacing calculated from substituting measured optical intensity of line spectrum of said pump light and said signal light, and nonlinear coefficient N2 and wavelength dispersion value D(fk) as variable numbers for the equation
P1(L)=xcex7xe2x80x2(2xcfx80N2/xcexp)2Pp2Psexp(xe2x88x92xcex1L)[{1xe2x88x92exp(xe2x88x92xcex1L)}/xcex1]
(where, xcexp is wavelength of pump light, Pp is optical intensity of line spectrum of pump light, Ps is optical intensity of line spectrum of signal light, xcex1 is transmission loss of optical fiber, and L is length of test optical fiber)
coincides with measured value of optical intensity of four wave mixing in each peak wavelength spacing, and obtaining nonlinear coefficient and/or wavelength dispersion value from the value of said substituted nonlinear coefficient N2 and wavelength dispersion value D(fk) as variable numbers.
Where, xcex7xe2x80x2 is four wave mixing (FWM) efficiency expressed by equation,
                              η          xe2x80x2                =                ⁢                  {                                                    P                1                            ⁡                              (                                  L                  ,                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          k                      xe2x80x2                                                                      )                                      ⁢                                          P                1                            ⁡                              (                                  L                  ,                                                            Δ                      ⁢                                              xe2x80x83                                            ⁢                      k                                        =                    0                                                  )                                              }                                        =                ⁢                              [                                          α                2                            /                              {                                                      α                    2                                    +                                                            (                                              Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                  k                          xe2x80x2                                                                    )                                        2                                                  }                                      ]                    [                      1            +                                          {                                  4                  ⁢                                      exp                    ⁡                                          (                                                                        -                          α                                                ⁢                                                  xe2x80x83                                                ⁢                        L                                            )                                                        ⁢                                                            sin                      2                                        ⁡                                          (                                              Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                  k                          xe2x80x2                                                ⁢                                                  L                          /                          2                                                                    )                                                                      }                            /                                                                      ⁢                                            {                                                exp                  ⁡                                      (                                                                  -                        α                                            ⁢                                              xe2x80x83                                            ⁢                      L                                        )                                                  -                1                            }                        2                    ]                    
where, xcex94kxe2x80x2 is phase adjustment factor expressed by equation,
xcex94kxe2x80x2=xcex94kxe2x88x92(2xcfx80N2/xcexp)(2Ppxe2x88x92Ps)[{1xe2x88x92exp(xe2x88x92xcex1Leff)}/xcex1Leff]
where, xcex94k is expressed by equation,
xcex94k=2|fpxe2x88x92fs|32xcfx80xcexs2D(fk)/c
(where, fp is frequency of line spectrum of pump light, fs is frequency of line spectrum of signal light, and xcexs is wavelength of line spectrum of signal light)
and Leff is effective length of test optical fiber expressed by equation.
Leff={1xe2x88x92exp(xe2x88x92xcex1L)}/xcex1
Further more, there is provided an apparatus for measuring wavelength dispersion value and/or nonlinear coefficient of optical fibers comprising, a first continuous wave light source for emitting a pump light, a second continuous wave light source for emitting signal light with different peak wavelength from said pump light and varying peak wavelength, a spectrum analyzer for measuring four wave mixing optical intensity generated in test optical fiber by emitting said pump light and said signal light to test optical fiber for each peak wavelength spacing, and for measuring optical intensity of a plurality of line spectrum constituting said pump light and a plurality of line spectrum constituting said signal light respectively, and an operating circuit for obtaining nonlinear coefficient and/or wavelength dispersion value by calculating so that maximum value of four wave mixing optical intensity in each spectrum wavelength spacing calculated from optical strength of line spectrum of said pump light and said signal light measured by said spectrum analyzer coincides with measured value of four wave mixing optical intensity in each spectrum wavelength spacing measured by said spectrum analyzer.
In the present invention, it is preferable that operating circuit is mounted in spectrum analyzer.